Is there a way to have an object show up only in a reflection?
Use the light paths node on the Earth to isolate the reflection light ray.



In this picture I have a sphere emitting light, but it is only seen on the reflection of the plane. On the right is the node set up for the Sphere. You can use that node set-up for the Earth, just replace the principled shader with your current node set up for the Earth.

Hope this helps!